The research on a gas sensor has been conducted for a long period of time and various types of gas sensors have been commercialized at present. Among them, there is a gas sensor using a semiconductor sensing material. In the gas sensor using the semiconductor sensing material, when a gaseous component is adsorbed onto the surface of a semiconductor or reacts with adsorbed gas such as oxygen which was adsorbed in advance, electrons are transferred between adsorbed molecules and the semiconductor surface, and as a result, conductivity and surface potential of the semiconductor are changed. Since the degrees of the changes depend on the concentration and measurement temperature of sensed gas, the degrees are used for a gas sensing principle.
A semiconductor gas sensor can be mass produced due to a simple structure and an easy process and are mounted in a personal portable terminal or used in a ubiquitous sensor network to be used in various services due to a small size and low power consumption as compared with an optical gas sensor or an electrochemical gas sensor through measurement of conductivity by a spectrum or ion mobility of measured air.
The semiconductor gas sensor of the related art includes a bulk type semiconductor gas sensor with a sensing material, a sensing electrode, and a heater for increasing an operational temperature in order to improve a sensing characteristic on a substrate made of alumina or quartz, or an MEMS type semiconductor gas sensor that is similar to the above gas sensor in other parts but removes a part with the heater through a method such as etching to thermally separate the corresponding part to reduce power consumption.
The bulk type semiconductor gas sensor is resistant to a sudden impact, while the bulk type semiconductor gas sensor has large power consumption in order to maintain a high operational temperature for improving the sensing characteristic, such that it is difficult to apply the bulk type semiconductor gas sensor to a portable terminal or a sensor node for a ubiquitous sensor network (USN) service.
The existing MEMS type semiconductor gas sensor has still lower power consumption than the bulk type semiconductor gas sensor by etching a substrate (primarily silicon) of a heat emitting resistor part and thermally separating the corresponding part. The method therefore is divided into a bulk micromachining method of etching a rear substrate while membranes and the heat emitting resistor part remain and a surface micromachining method of etching only a part of the rear substrate while some of the membranes and the heat emitting resistor part remain.
In the former case, since the membranes are connected with the substrate without a hole, the membranes have relatively mechanical strength, but a portion having a size corresponding to the thickness of the substrate needs to be etched, and as a result, process cost is increased. In the latter case, since only a part of the thickness of the rear substrate is etched while some of the membranes and the heat emitting resistor part remain, the process cost is relatively low and the rear surface of the substrate is closed, and as a result, it is easy to handle elements, but since the membranes are connected with the substrate while the membranes have the holes, the membranes have a relatively weak structural characteristic. Therefore, both the structures are relatively weak to the sudden impact.
However, an MEMS type semiconductor gas sensor is required, which is structurally, mechanically, and electrically stable even with respect to the sudden impact while having lowest amount of power consumption in order to be used for various services with being mounted on the portable terminal or the ubiquitous sensor network.